Polytetrafluoroethylene (PTFE) has excellent chemical resistance, weather resistance and mechanical strength, among others, and, therefore, is utilized in the form of molded articles in various fields.
As a method of molding PTFE, there may be mentioned, for example, the technique of extrusion molding which comprises paste extrusion molding of a PTFE fine powder.
Industrially, the PTFE fine powder to be used in paste extrusion is preferably one excellent in paste extrudability, for example one for which the ratio between the cylinder cross-sectional area and the die opening cross-sectional area (i.e. reduction ratio [RR]) in the extruder on the occasion of paste extrusion is high and for which the extrusion pressure is low.
As for PTFE fine powders improved in paste extrudability, there have been proposed, for example, a particulate TFE resin each particle of which has a particle shell resulting from copolymerization of a specific modifier and which can be extruded at a RR of 1600 (cf. e.g. Patent Document 1: Japanese Patent Publication (Kokoku) S37-4643), and a modified tetrafluoroethylene polymer with a particle shell and an outermost particle shell respectively modified with chlorotrifluoroethylene [CTFE] at different levels of modification (cf. e.g. Patent Document 2: Japanese Kokai (Laid-open) Publication S51-36291).
However, such a particulate TFE resin still has problems, namely the extrusion pressure is still high and the thermal stability is unsatisfactory; and the modified tetrafluoroethylene polymer also has a problem in that the extrusion pressure is still high.
As for PTFE fine powders improved in paste extrudability, there have also been proposed, for example, one each particle of which has a particle core comprising fluoroalkyl vinyl ether-modified PTFE and a particle shell comprising CTFE-modified PTFE and which can be extruded at a RR of 2000 (cf. e.g. Patent Document 3: Japanese Kokai Publication S63-56532), and one each particle of which has a particle core comprising modified PTFE resulting from copolymerization with a fluoroalkylethylene in an amount smaller than 0.5% by weight relative to the whole particle and which can be extruded at a RR of 2000 (cf. e.g. Patent Document 4: Japanese Kokai Publication S60-42446 and Patent Document 5: Japanese Kokai Publication H05-186532). However, these PTFE fine powders have problems, namely they are unsatisfactory from the thermal stability and extrusion pressure viewpoint.
Further, as a PTFE fine powder improved in paste extrudability, there has been proposed one which is obtained by subjecting a perfluoroalkyl vinyl ether having a specific structure and TFE to copolymerization and, after consumption of at least 80% of TFE to be polymerized, introducing a chain transfer agent into the reaction system (cf. e.g. Patent Document 6: WO 00/02935). However, such a PTFE fine powder has a problem, namely it is unsatisfactory from the extrusion pressure viewpoint.
In certain instances, PTFE fine powders are preferably processed into tubes excellent in transparency according to the respective purposes. Mention is made of the transparency of the molded articles obtained from PTFE fine powders in some prior art documents (cf. e.g. Patent Document 3) but no specific data are described in any document.